I do the majority of my shooting in very low light situations without a flash. I started out shooting at night with a (then brand new) Olympus e-510 and a Pan/Leica 25mm f1.4. I know, great lens choice, not best choice for a low-light camera, but that's what I happened to own when what became a three year nightime photo-shoot opened up for me. Those photos BTW have been published in a variety of newspapers & magazines, as well as having two gallery shows of them. Now I'm shooting different very low-light subjects with an OM-D and the m4/3 version of the PL 25 1.4, and I couldn't be happier. Why? A big part is because of the depth of field that combo gives me.

Like many others, I too like to seperate my subjects from their background a bit by using the selective focus which wide apertures afford. But often, I don't want to obliterate all the background, or foreground, definition, just soften it enough to tell my viewers wher to plant their gaze in the picture. So, if I'm shooting in a dimly lit bar or a city street at midnight, my OM-D produces "just right" images at f1.4 and an ISO of 3200, typically yielding a shutter speed of between 1/30 and 1/80 sec, depending on how dark the lighting is.

Of course, I could shoot FF at 2.8, but then I'd also be shooting at an ISO of 12800 for the same shutter speed. Even as good as FF images are, the OM-D at 1/4 the ISO will always look better.

...about your last sentence above. It would be interesting to see a comparison between a modern FF DSLR with a 50 / 1.4 at f/2.8 and the same shutter speed, and see which comes out ahead. I am far from convinced that it would be the EM5 (which is not to say that I am convinced it will be the FF system).

On what grounds are you not convinced? As I know you are well aware, any current FF camera would lose out to the E-M5 for DR/shadow noise in this comparison (except the D4 which would break even).

1Dx will obliterate EM5 @ high ISO.

Not if the 1Dx is using two EV higher ISO (e.g., 3200 versus 12800) for equal DoF as in the comparisons we are talking about in this thread. In that case, the E-M5 will have a slight edge.